In a carriage printer, such as an inkjet carriage printer, a printhead is mounted in a carriage that is moved back and forth across the region of printing. To print an image on a sheet of paper or other print medium, the medium is advanced a given nominal distance along a media advance direction and then stopped. Media advance is typically done by a roller and the nominal distance is typically monitored indirectly by a rotary encoder. While the medium is stopped and supported on a platen, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as marks are controllably made by marking elements on the medium—for example by ejecting drops from an inkjet printhead. Position of the carriage and the printhead relative to the print medium is precisely monitored directly, typically using a linear encoder. After the carriage has printed a swath of the image while traversing the print medium, the medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
In order to produce high quality images, it is helpful to provide information to the printer controller electronics regarding the printing side of the recording medium, which can include whether it is a glossy or matte-finish paper. Such information can be used to select a print mode that will provide an optimal amount of ink in an optimal number of printing passes in order to provide a high quality image on the identified media type. It is well-known to provide identifying marks or indicia, such as a bar code, on a non-printing side of the recording medium to distinguish different types of recording media. It is also well known to use a sensor in the printer to scan the indicia and thereby identify the recording medium and provide that information to the printer control electronics. U.S. Pat. No. 7,120,272, for example includes a sensor that makes sequential spatial measurements of a moving media that contains repeated indicia to determine a repeat frequency and repeat distance of the indicia. The repeat distance is then compared against known values to determine the type of media present.
U.S. Pat. No. 8,033,628 discloses the use of a backside media sensor to read a manufacturer's code for identifying media type. In this approach light from a light source is reflected from the backside of the media and received in a photosensor while the print media is being advanced past the photosensor. A source of potential unreliability in interpreting the signals is that media can slip during advance past the photosensor.
U.S. Pat. No. 8,118,390 discloses reflecting light from the backside of the media using an optical path between the carriage and a media input location and sensing the manufacturer's code by a sensor while the media is in the media input location. Such an approach is compatible with media travel paths in which the backside of the media is viewable when the media is in the media input location. However, this is difficult in some other types of media travel paths, especially where the printing side of the media faces outward away from the stack of media.
Co-pending U.S. Patent Application Publication 2011/0096118 discloses an inkjet printer having a paper path where the backside of the print medium is not visible from the carriage throughout the entire media travel path. By using infrared light sources disposed in the platen to transmit light through the print medium, a sensor that is mounted on the carriage can be used to detect the manufacturer's code and identify the media type using transmitted light.
Consequently, a need exists for an apparatus and method for identifying the type of print medium from the manufacturer's code on the backside of the print medium using reflected light for a media travel path and the precisely monitored motion of the carriage for types of media travel paths in which the backside of the print medium is not visible from the carriage throughout the entire media travel path.